SONOMA COUNTY, Calif.--When it comes to flying, being green isn't easy.

Here at NASA's second annual General Aviation Challenge this weekend, one of the main prizes was the so-called "green prize," which challenged two-seater planes to fly a 400-mile-long course logging at least 30 miles to the gallon. None of the four planes entered won the $50,000 prize; the best attempt achieved 28.8 miles per gallon. (NASA's ultimate goal is to get the green prize to at least 100 miles per hour and the equivalent of 100 miles per gallon.

The shoe-in to win the green prize--a team with a novel "omnivore" biodiesel engine capable of flying on french fry oil--pulled out of the race at the last minute because of a mechanical problem.

"Ironically, our biggest advance this year was with a plane that didn't fly," said Andy Petro, head of NASA's Centennial Challenges, a series of government-sponsored competitions that support space exploration and aviation technologies in private industry. Petro was referring to the team Kantankeros Katana's dual biodiesel and gas engine, which could conceivably run on everything from automobile gasoline to kerosene. Contest organizers have said that a biodiesel-engine plane could go 900 miles on 25 gallons.

Despite this year's letdown, NASA plans to retool its contest next year to focus primarily on fuel-efficient aviation, according to Petro.

"Next year, we'll be emphasizing fuel efficiency as the ultimate goal with planes that maintain the characteristics of practical aircraft," he said. The prize money will be up to $400,000 at next year's event.

The Cafe Foundation, a nonprofit group of flight test engineers, hosted the aviation challenge here at the Charles M. Schulz Sonoma County Airport in California. The races ended Saturday and the winners were announced Sunday.

Apart from the green prize, NASA awarded about $100,000 of its total $300,000 allotted for the second annual challenge, less money than what was doled out last year. One of the biggest prizes it granted was $50,000 for aircraft safety to the lone returning competitor, the Slovenian-built Pipistrel known as Virus. The plane, which was the big
, had added such precautions as a cabin integrated with Kevlar and an installed ballistic parachute system, or a deployable rocket that would launch a parachute 100 feet above the plane in the event of an emergency.

NASA also awarded prizes for competitors' ability to fly quietly--a characteristic that's of increased importance as planes land nearer to communities. The Pipistrel, for example, used a carbon-fiber propeller on its aircraft this year to reduce its noise by at least 10 percent, but that shift cut the plane's fuel-efficiency by as much as 50 percent. The "Prius of airplanes," the 100-horsepower-engine Pipistrel typically can go as fast as 170 mph and get 50 miles to the gallon.

Overall, NASA has staked a total of $2 million for the five annual aviation contests. (Last year, NASA
$250,000 in prize money.) In 2005, the challenges were originally devised as a way to advance so-called personal air vehicles, or PAVs, so that in the future, people could beat gridlock by commuting in a two-seater plane with zero emissions. NASA backed away from that Jetsons-like view of the future this year by changing the contest to promote general aviation technologies, such as the development of tech for unmanned aircraft.

One of the keys to a vision of PAVs is energy efficiency. The CAFE Foundation has said that it expects to see its first electric airplane competitor by 2009--and at least one of the contestants is on track to deliver on that expectation.

Pipistrel plans to begin selling the first commercially produced, two-passenger electric aircraft, the Taurus Electro by the end of the year. On one charge, the Taurus glider is expected to travel as much as 1,000 miles in a day. It will cost $133,000. (Pipistrel's Virus costs about $110,000.)

Geoff Stevenson, a mechanical engineer and owner of the biodiesel plane, a modified Diamond DA20-A1, said that he and his brother had to pull their craft from the contest because its air inlet temperature sensor was broken. It wasn't communicating with the on-board computer. They also had yet to test fly the plane. Still, he thought it would have been competitive in the green prize and they plan to race next year.

"The push to develop a diesel engine is worthy because of the limited availability of aviation leaded fuels," Stevenson said. He added that he and his brother spent about $15,000 on the 12 year old air frame, and another $3,500 to develop the engine.

One of the more encouraging signs of this year's race, Petro said, was that engineers from the Slovenian sport-plane company Pipistrel showed up and modified their craft to specifications of the challenge. For example, they added insulation to the cabin to cut down on noise and installed an electronic engine and flight management system to improve the plane's safety. The flight management system, which draws on GPS and satellite systems, would act as a guide to the pilot in the event of bad weather.

Part of the goal of the aviation challenge is to get the aircraft industry thinking about innovations in fuel-efficiency, safety, and noise reductions.

"We don't expect revolutionary advances at this stage. But we are making incremental steps in that direction," Petro said.